Method and apparatus for non-disruptive telecommunication loop condition determination

ABSTRACT

In one embodiment, a low cost, simple circuit for detecting an off-hook condition of a telecommunication line comprising tip and ring signal lines is provided. The circuit comprises a voltage divider for coupling between the tip and ring lines without an intervening transistor and having a node at which is presented a scaled version of a voltage across the voltage divider. The circuit further comprises a transistor having a control terminal coupled to the node, a first current flow terminal coupled to a voltage source, and a second current flow terminal coupled to an output terminal, wherein the output terminal bears a value that is indicative of a voltage across the tip and ring lines and thus whether the telecommunication line is off-hook.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending application Ser. No.11/534,918 filed Sep. 25, 2006, which is a continuation of applicationSer. No. 09/605,953 filed Jun. 28, 2000 and issued as U.S. Pat. No.7,113,587, the teachings of both of which are incorporated herein byreference.

FIELD OF THE INVENTION

Embodiments of the invention pertain to telecommunication systems, andmore particularly, to determining the condition of a telecommunicationloop circuit, such as determining whether any equipment coupled to theloop is off-hook.

BACKGROUND OF THE INVENTION

In recent years, there has been substantial expansion in the number andtype of telecommunications equipment in common use in households andoffices. For instance, it is not unusual for a subscriber to havemultiple pieces of telecommunications equipment coupled to a singletelephone subscriber loop, e.g., a single tip and ring wire pair. Forinstance, a household might have a telephone, a facsimile machine, acomputer using a modem, and an answering machine hooked up to a singlesubscriber loop. Other than for voice communications using a telephone,it is possible for only one piece of equipment to use the loop at anygiven time. If a second piece of equipment goes off-hook while a firstpiece of equipment is using the loop to transmit or receive data, thenoise and change in loop voltage due to the second piece of equipmentgoing off-hook can cause data errors in connection with the first pieceof equipment. Accordingly, many automated telecommunication apparatusessuch as fax machines and modems are designed to detect the condition ofthe telephone line to which they are coupled before they go off-hook.Thus, for instance, if a facsimile machine coupled to a subscriber loopis receiving a facsimile (such that the line is off-hook), a computermodem having this feature will first check if the line is already in anoff-hook condition before going off-hook itself and suppress an attemptto go off-hook if it detects that the subscriber loop is alreadyoff-hook (i.e, that another piece of equipment on the same line isoff-hook).

While there are many ways to determine whether a subscriber loop circuitis off-hook, probably the simplest way is to determine the DC voltage onthe line. In the United States, subscriber loops are biased toapproximately 48 volts DC (Direct Current) when the line is not in use(i.e, when no equipment coupled to that line is off-hook). If atelecommunication device on the line is off-hook, then the voltage dropstypically to somewhere in the range of 20 volts or less.

In the prior art, telecommunication equipment manufacturers haveutilized voltage comparator circuits to detect the DC voltage across tipand ring of a subscriber loop and to use the comparator output as anon-hook/off-hook indicator signal. FIG. 1 is an exemplary circuit of theprior art. In the circuit of FIG. 1, a full wave rectifier 12 is coupledacross tip 14 and ring 16. The tip′ output 14 a from the rectifier iscoupled to a voltage divider 17 comprising resistors 18 and 20. Thecommon node 22 of the voltage divider is coupled to one input of acomparator 24. The other input of the comparator is coupled to areference voltage 26. The output of comparator 24 is coupled through ahigh voltage barrier circuit, such as optical coupler 28 to a digitalsignal processor (DSP) 25. A less expensive electrical (as opposed tooptical) coupling circuit, whether inductive or capacitive, would notfunction well in this application. Particularly, the signal across tipand ring can change too slowly to be distinguished from noise by anelectrical coupling circuit.

Of course, it will be understood by those of skill in the art thatsubstantial additional circuitry is coupled across tip and ring that isnot illustrated in FIG. 1 in order to provide the functionality of thecircuit (e.g., to send and receive facsimiles) and that FIG. 1 merelyshows the loop status detection circuitry.

The reference voltage and values of the resistors 18 and 20 in theresistor voltage divider 17 are selected so that the comparator outputchanges state somewhere between the on-hook voltage (approximately 48volts in the U.S.) and the off-hook voltage (approximately 20 volts inthe U.S.). Accordingly, for example, the reference voltage and dividernetwork resistor values can be selected so that the switching point ofthe comparator is at approximately 30 volts across tip and ring. Thedigital signal processor is programmed to disable the telecommunicationapparatus from going off-hook when the voltage across tip and ring isless than 30 volts. Otherwise, the DSP allows the apparatus to operatenormally.

Except for the DSP, the circuit shown in FIG. 1 is specificallydedicated to the aforementioned feature. The DSP typically would be aDSP that already exists in the circuit for performing some or all thefunctions of the actual device (e.g., facsimile machine) and wouldmerely have additional functionality built into it for receiving thecomparator output signal and selectively enabling/disabling the devicefrom going off-hook responsive thereto.

The telecommunication loop detection circuit itself must not disrupt theloop when checking the loop voltage. To do so would, of course, defeatits very purpose.

Accordingly, it is an object of the present invention to provide atelecommunication loop condition detector that is simpler and lower incost than prior art detectors. The telecommunication device must notdisturb any call function with any audible noise injection and must notsignificantly alter the loop impedance.

SUMMARY OF THE INVENTION

Embodiments of the invention provide a low cost, simple, circuit fordetecting the condition of a telephone line. Particularly, an embodimentof the inventive circuit utilizes an existing low poweranalog-to-digital converter that is already incorporated into thetelecommunication device and used for other functions such as caller IDand ring detection. The additional circuitry comprises a voltage dividercoupled between tip and ring and a transistor having its control input(e.g., gate or base) coupled to the common node of the voltage divider,one of its current flow terminals (e.g., collector, emitter, drain, orsource) coupled to the analog input of the analog-to-digital converter,and the other current flow terminal coupled to ground. The A/D converteris also coupled to tip and ring, respectively, through a pair ofcapacitors for detecting the AC voltage on the line for purposes ofcaller ID and/or ring detection.

The resistors of the voltage divider are proportioned such that thecommon node voltage of the divider is above the threshold voltage of thetransistor, thus turning it on, when the voltage across tip and ring isat the on-hook voltage of approximately 48 volts, and is below thetransistor threshold voltage, thus turning it off, when the voltageacross tip and ring is at the off-hook voltage of approximately 20volts. Accordingly, when the transistor is on, the A/D converter inputis driven to ground. When it is off, the A/D converter input goes to itsself biased voltage. Accordingly, the digital output of the A/Dconverter is an indication of the voltage on the line and thus whetherit is on-hook or off-hook. The output of the A/D converter can becoupled to a digital signal processor that disables the device fromgoing off-hook if the A/D converter detects that the loop is off-hook.

In an alternative embodiment, the voltage across one resistor of aresistor voltage divider that is coupled between tip and ring isprovided to the analog input of the low power analog-to-digitalconverter through the current flow terminals of one or more transistorsso that the A/D converter receives a scaled version of the actual tip toring voltage rather than simply a two state on-hook/off-hook signal. TheDSP may use the specific loop voltage information provided in thisembodiment to determine additional information about the loop.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a telecommunication line status detectioncircuit of the prior art.

FIG. 2 is a circuit diagram showing a telecommunication line statusdetection circuit in accordance with the first embodiment of the presentinvention.

FIG. 3 is a circuit diagram showing a telecommunication line statusdetection circuit in accordance with a second embodiment of the presentinvention.

FIG. 4 is a circuit diagram of one particular data access arrangementinto which an embodiment of the present invention has been incorporated.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention is a low-cost, simple, circuitfor detecting line voltage across tip and ring of a telecommunicationsubscriber loop that primarily utilizes circuitry that is commonlyalready incorporated in a data access arrangement (DAA) oftelecommunication equipment. The circuit can be used to detect whetherthe line is in an off-hook condition and particularly can be used fordisabling equipment from going off-hook if the line already is in use(i.e., off-hook) by another piece of telecommunication equipment. Apreferred embodiment of the invention is particularly adapted for use intelecommunication equipment incorporating the CSP 1035 Silicon DAAmanufactured by Lucent Technologies, Inc. of Murray Hill, N.J., theassignee of the present application. However, it will be obvious tothose of skill in the telecommunication equipment field that embodimentsof the invention can be utilized with other DAA designs.

A DAA commonly includes circuitry for performing various functionsincluding ring detection, DC loop hold, hook control and pulse, parallelphone sense and data/voice relay. Some or all of these functions may beperformed by a programmed DSP. Some DAAs includes a low poweranalog-to-digital converter coupled to receive a differential signalfrom the tip and ring line pair so that a DSP can perform functions inconnection with caller ID and ring detection.

The circuitry of one embodiment of the present invention is shown inFIG. 2 and appears within dashed box 30 in FIG. 2. FIG. 2 alsoillustrates some of the circuitry that already commonly exists in a DAAand particularly the circuitry that is relevant to the operation of theinventive circuit 30. The detection circuit 30 comprises resistors R1and R2, diode D1 and transistor Q1. In particular, resistors R1 and R2comprise a resistor voltage divider coupled between tip′ and ring′. Notealso that ring′ is coupled to analog ground. The common node N1 betweenresistors R1 and R2 is coupled to the control terminal (the gate, in thecase of a MOSFET) of transistor Q1. Node N1 also is coupled to a controlsignal line 40 through diode D1.

This control signal line 40 pre-exists in many DAAs and is designed toremain at a logic low level until just before the equipment attempts togo off-hook, at which time the control signal goes to logic high. Aswill become clear from the description below, operation of embodimentsof the present invention may interfere with other functions of theequipment, such as caller ID and ring detection. Accordingly, thiscontrol line is used to disable operation of the inventive detectioncircuit until just before the equipment attempts to go off-hook so that,for instance, caller ID can operate without interference until theequipment is ready to go off-hook.

The current flow terminal of transistor Q1 (source and drain in the caseof a MOSFET) are coupled between ring′ (ground) and one of the inputs ofdifferential, low power, A/D converter 36.

The low power A/D converter is a differential converter that converts adifferential analog voltage input across its two inputs to a digitalvalue. Accordingly, the “+” and “−” inputs of the converter are coupledto tip and ring of the telephone line through capacitors C1 and C2,respectively.

Capacitors C1 and C2 block DC current on tip and ring from the A/Dconverter so that only the AC current across tip and ring reaches theA/D converter through those paths. The digital output of the A/Dconverter is coupled to a digital signal processor that reads the ACinformation and performs caller ID and ring detection functions as knownin the prior art.

Tip and ring are also coupled to a full wave rectifier BR1 to producetip′ and ring′ signals. The full wave rectifier is employed simply toassure that tip′ is always positive compared to ring′. Specifically, itis possible that the polarity of tip and ring can be reversed. RectifierBR1 assures that tip′ is always more positive than ring′.

When the control signal is at logic low, node N1 is essentially coupledto ground thus keeping transistor Q1 turned off. With transistor Q1turned off, detection circuit 30 has no affect on the analog input ofA/D converter 36. Accordingly, the AC signals from tip and ring arereceived by the A/D converter 36 without interference, which signals canbe used for caller ID, ring detection and similar functions.

When the signal line goes high, diode D1 is essentially open circuitedand the resistor divider formed by R1 and R2 will selectively turntransistor Q1 on or off. In particular, resistors R1 and R2 are ratioedrelative to each other so that the voltage at node N1 is greater thanthe threshold voltage of Q1 when the voltage between tip′ and ring′ isat the on-hook voltage of the line and will be below the thresholdvoltage of transistor Q1 when the voltage between tip′ and ring′ is atthe off-hook voltage level.

As previously mentioned, the standard on-hook DC voltage across tip andring for a subscriber loop in the United States is approximately 48volts, while standard off-hook DC voltage across tip and ring isapproximately 20 volts. Accordingly, resistors R1 and R2 can have aratio relative to each other so that the common node voltage will be thethreshold voltage of transistor Q1 when the voltage between tip and ringis anywhere between just above 20 volts and just below 48 volts.

The inputs of the A/D converter are both biased to the common modevoltage. Accordingly, when transistor Q1 is turned off, the DC voltageacross the differential inputs of A/D converter 36 is approximately 0volts. However, when transistor Q1 is turned on, the − input terminal ofthe A/D converter is driven to ground through the current flow terminalof transistor Q1 while the + input terminal remains at common modevoltage. Accordingly, when transistor Q1 is turned on, A/D converter 36detects one half full scale voltage.

Accordingly, in operation, when the control signal 40 goes high, thusopen circuiting diode D1, the A/D converter will detect 0 volts acrossits differential inputs if the line is off-hook (and thus transistor Q1is turned off). However, if the line is on-hook, transistor Q1 is turnedon so that A/D converter 36 will detect one half full scale voltageacross its differential inputs.

The output of the A/D converter 36 is coupled to the digital signalprocessor 25. The digital signal processor 25 is programmed to preventthe equipment from going off-hook if it receives approximately 0 voltsat this instant and to allow the equipment to go off-hook if it receivesone half full scale voltage at this instant.

Accordingly, the inventive circuit provides a non-disruptive linecondition detection function with a minimum of additional circuitry. Inthe embodiment shown in FIG. 2, for instance, the circuit adds only tworesistors, a diode and a transistor to the DAA.

FIG. 3 illustrates an alternative embodiment 50 of the present inventionin which the A/D converter detects a scaled version of the actualvoltage across tip and ring rather than merely a two state(on-hook/off-hook) signal. The alternative circuit is shown in box 50.It comprises resistors R3 and R4, diode D2 and transistors Q2 and Q3.Resistors R3 and R4 form a resistor divider network coupled between tip′and ground, just as in the FIG. 2 embodiment, except that diode D2 iscoupled between the bottom of resistor R4 and ground. Diode D2 assuresthat the source terminals of transistors Q2 and Q3 are referenced toground. In essence, diode D2 acts as a regulator, keeping the voltage atthe source terminal of transistor Q3 at or above 0.7 volts (the biasvoltage of the diode).

Transistor Q2 has its current flow terminals coupled between the commonnode N2 between resistors R3 and R4 and one of the differential inputsof the A/D converter 36. Transistor Q3 has one of its current flowterminals coupled between resistor R4 and diode D2 and its other currentflow terminal coupled to the other differential input of the A/Dconverter. The control terminals (gates) of both transistors Q2 and Q3are coupled to the aforementioned control signal 40.

In operation, transistors Q2 and Q3 are turned off until the controlsignal 40 goes high just before the device attempts to go off-hook. Withtransistors Q2 and Q3 turned off, circuit 50 will have no effect on theoperation of A/D converter 36. However, when the control signal 40 goeshigh, transistors Q2 and Q3 will be turned on. Accordingly, A/Dconverter 36 will detect the voltage across resistor R4 at its twodifferential input terminals. Since resistors R3 and R4 (and diode D2)form a voltage divider across tip′ and ring′, this voltage is simply ascaled version of the voltage between tip′ and ring′. Accordingly, theDSP which receives the output of the A/D converter will receive a scaledversion of the DC line voltage and can react accordingly. Thus, with theaddition of one extra transistor over the embodiment of FIG. 2, theembodiment of FIG. 3 provides the actual tip to ring DC voltage to theDSP. The DSP can use this more specific information about the DCcondition of the loop as needed.

FIG. 4 is a partial block, partial schematic diagram of an exemplary DAA100 incorporating an embodiment of the present invention. The circuitryof the second embodiment of the present invention is shown in dashed box50. The aforementioned DSP and low power A/D converter are shown at 25and BR1, respectively. The DAA 100 further includes a digital bit outputcontroller 107 which is the source of the aforementioned control signal40 as well as other control signals in the DAA. The DAA further includesa full power receive A/D converter 101 and transmit D/A converter, bothof which couple to the tip and ring line pair through circuitry 105 forconditioning signals. Circuitry 105 performs various function, includinghook switch line modulation, shunt regulation A/D and D/A interfacing.The DSP 25 receives the digital output data from the A/D converter 36through a digital transmitter, shown as part of circuit 109, and a highvoltage interface circuit 111.

Since the DC line voltage is encoded by the low power A/D converter 36,the high voltage interface circuit may be a less expensive and complexelectrical high voltage interface circuit and need not be optical. Inessence, the A/D converter modulates the DC voltage signal. Thus, thereis no slow moving voltage that must go through the high voltageinterface that would preclude the use of an electrical, as opposed to anoptical, high voltage interface.

The DSP also sends digital information to various circuits through thehigh voltage interface circuit 111 and a digital data receiver portionof circuit 109. For instance, the DSP 25 communicates with the digitaloutput bit controller 107 through circuits 111 and 109 and thus cancontrol signal 40 as needed. The DSP 25 includes algorithms forperforming various functions, including line status detection inaccordance with an embodiment of the present invention, ring detection,and caller I.D. functions based on the signals that are received throughthe low power A/D converter 36. DAA 100 of FIG. 4 is merely an exemplaryDAA employing an embodiment of the present invention. It should be clearto those of skill in the related arts that embodiments of the inventioncan be employed in other DAAs also.

Having thus described a few particular embodiments of the invention,various alterations, modifications, and improvements will readily occurto those skilled in the art. Such alterations, modifications andimprovements as are made obvious by this disclosure are intended to bepart of this description though not expressly stated herein, and areintended to be within the spirit and scope of the invention.Accordingly, the foregoing description is by way of example only, andnot limiting. The invention is limited only as defined in the followingclaims and equivalents thereto.

We claim:
 1. A circuit for detecting whether a telecommunication line isoff-hook, said telecommunication line comprising tip and ring signallines, said circuit comprising: a voltage divider for coupling betweensaid tip and ring lines without an intervening transistor and having anode at which is presented a scaled version of a voltage across saidvoltage divider; and a transistor having a control terminal coupled tosaid node, a first current flow terminal coupled to a voltage source,and a second current flow terminal coupled to an output terminal,wherein said output terminal bears a value that is indicative of avoltage across said tip and ring lines and thus whether saidtelecommunication line is off-hook.
 2. The circuit of claim 1 wherein:said voltage divider comprises a first resistor having a first terminalfor coupling to said tip line and a second terminal coupled to said nodeand a second resistor having a first terminal for coupling to said ringline and a second terminal coupled to said node.
 3. The circuit of claim2 further comprising: an analog to digital converter having an analoginput and a digital output, said analog input coupled to said outputterminal wherein said analog input is biased to a voltage different thana voltage of said voltage source.
 4. The circuit of claim 3 furthercomprising: a processor coupled to said digital output of saidanalog-to-digital converter, said processor adapted to determine whethersaid telecommunication line is off-hook based on a signal on saiddigital output of said analog-to-digital converter.
 5. A circuit fordetecting whether a telecommunication line is off-hook, saidtelecommunication line comprising tip and ring signal lines, saidcircuit comprising: a voltage divider for coupling between said tip andring lines and having first and second nodes across which appears ascaled version of a voltage across said tip and ring lines; adifferential analog-to-digital converter having first and second analoginput terminals and a digital output terminal; a signal line forselectively enabling said circuit when said signal is in a first stateand disabling said circuit when said signal is in a second state; afirst transistor having a control terminal coupled to said signal lineand first and second current flow terminals coupled between said firstnode and said first input terminal of said analog-to-digital converter;a second transistor having a control terminal coupled to said signalline and first and second current flow terminals coupled between saidsecond node and said second input terminal of said analog-to-digitalconverter, wherein: when said signal line is in said first state, saidanalog-to-digital converter receives a scaled version of the voltageacross said tip and ring lines, and, when said signal line is in saidsecond state, said analog-to-digital converter receives no signal fromsaid voltage divider.
 6. The circuit of claim 5 wherein: said voltagedivider comprises a first resistor having a first terminal coupled to afirst one of said tip and ring lines and a second terminal coupled tosaid first node and a second resistor having a first terminal coupled tosaid first node and a second terminal coupled to said second node. 7.The circuit of claim 6 further comprising: a processor coupled to saiddigital output of said analog-to-digital converter, said processoradapted to determine whether said telecommunication line is off-hookresponsive to a signal on said digital output of said analog-to-digitalconverter.
 8. A telecommunication apparatus for coupling to atelecommunication link, said telecommunication link comprising tip andring lines that are biased to a first voltage when said link is on-hookand a second voltage when said link is off-hook, said apparatuscomprising: a processor; a first circuit for taking said apparatusoff-hook so that said apparatus may receive or transmit information viasaid telecommunication link; a voltage divider for coupling between saidtip and ring lines and having first and second nodes across whichappears a scaled version of a voltage across said tip and ring lines; adifferential analog-to-digital converter having first and second analoginput terminals and a digital output terminal; a signal line forselectively enabling said circuit when said signal is in a first stateand disabling said circuit when said signal is in a second state; afirst transistor having a control terminal coupled to said signal lineand first and second current flow terminals coupled between said firstnode and said first input terminal of said analog-to-digital converter;a second transistor having a control terminal coupled to said signalline and first and second current flow terminals coupled between saidsecond node and said second input terminal of said analog-to-digitalconverter, wherein: when said signal line is in said first state, saidanalog-to-digital converter receives a scaled version of the voltageacross said tip and ring lines, and, when said signal line is in saidsecond state, said analog-to-digital converter receives no signal fromsaid voltage divider; and said processor is adapted to disable saidfirst circuit responsive to said analog-to-digital converter receivingsaid second voltage and enable said first circuit responsive to saidanalog-to-digital converter receiving said first voltage.
 9. Theapparatus of claim 8 wherein: said voltage divider comprises a firstresistor having a first terminal coupled to a first one of said tip andring lines and a second terminal coupled to said first node and a secondresistor having a first terminal coupled to said first node and a secondterminal coupled to said second node.
 10. The apparatus of claim 8further comprising: said processor is coupled to said digital output ofsaid analog-to-digital converter and adapted to determine whether saidtelecommunication line is off-hook responsive to a signal on saiddigital output of said analog-to-digital converter.
 11. A method fordetecting whether a telecommunication line is off-hook without affectingthe line impedance, said telecommunication line comprising tip and ringsignal lines, said method comprising the steps of: modulating a DCvoltage that appears across said tip and ring lines; passing saidmodulated DC voltage through an electrical high voltage interfacecircuit; and determining whether said telecommunication line is off-hookas a function of said modulated DC voltage.
 12. The method of claim 11wherein modulating a DC voltage comprises converting said DC voltageappearing across said tip and ring lines from analog to digital.
 13. Themethod of claim 11 further comprising scaling said analog DC voltageappearing across said tip and ring lines before step (1).
 14. The methodof claim 13 further comprising selectively enabling said DC voltageappearing across said tip and ring lines to be modulated.
 15. The methodof claim 13 wherein said DC voltage appearing across said tip and ringlines is scaled by a voltage divider.
 16. The method of claim 11 whereindetermining whether said telecommunication line is off-hook comprisescomparing said modulated DC voltage to a reference value.
 17. The methodof claim 11 wherein determining whether said telecommunication line isoff-hook is performed by a digital signal processor.